dash/ash

Positional parameters are scoped to the function. So the set -- construct only changes the function-local argument vector. In the following example, note that invocation of foo doesn't reset the positional parameters of 1 2 3 4 in the outer scope.

foo() {
set -- A B C

printf "$# -> $*\n"
}

printf "$# -> $*\n"
set -- 1 2 3 4
foo
printf "$# -> $*\n"

Also, per POSIX: "Positional parameters are initially assigned when the shell is invoked (see sh), temporarily replaced when a shell function is invoked (see Function Definition Command), and can be reassigned with the set special built-in command."

Obviously this isn't a complete substitute for arrays, neither alone nor in tandem with other alternatives.

But my point is that people use Bash features without understanding the alternatives, and without appreciating the cost of Bash. Bash is a great interactive shell. But if you're serious about shell programming, one should learn POSIX syntax, as well as sed(1), awk(1), and the nuances of other utilities. This isn't hard because the POSIX documentation is infinitely more clear and concise than implementation documentation. A good way to practice is with Solaris, because Solaris' utilities are often annoyingly anachronistic (e.g. grep doesn't even have a recursive mode), but dash isn't a bad place to start, either. And when you do choose to use extended functionality, do so knowingly, having assessed the costs and benefits.